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Principal quantum numbers, which are symbolized by "n", start with n equals one followed by n equals 2 and so on. The second orbit, or n equals 2, is larger than the first. Energy has to be absorbed by an electron for it to jump from the first orbit to the second, so the second orbit contains more energy.

The one-dimensional Bohr model introduced the principal quantum number. Three quantum numbers, "n", "l" and "m" were introduced by Schrodinger's three dimensional model. The second two variables refer to angular and magnetic quantum numbers respectively. The former represents the orbital shape, while the latter represents the orbital orientation in space.